@batch: Support for some non-batchable arguments to group by

[chr1st1ank]

The @Batch decorator could support arguments which are not batched, but are rather used to group the calls by. Calls with the same value in these arguments can be batched together.

This is for the case that these other arguments are configuration options of any kind, e.g.:

def f(x: List, with_preprocessing = True):
    ...